Prior to an account of the prior art, some terms that relate to the description hereinbelow will be defined.
Charging information for a call, i.e. a tariff, refers to price information that relates to the charging rate or to the number of pulses in a pulse train, or both. This information must be available for calculation of the price for a call. The price information may be given as an amount per charging rate or an amount per pulse, for example. A charging rate in time charging again relates to the time between two successive metering pulses.
In modern telephone networks, it is possible to keep subscriber-specific accounts into which the subscriber may deposit money and thereby obtain prepaid call time. When the subscriber makes a call, the system decrements his account balance on the basis of the connection time spent. One advantage of such a service from the point of view of the operator is that the operator can avoid subscriber-related indebtedness, as it is possible not to allow calls when there are not sufficient funds in the account. On the other hand, from the point of view of the subscriber it is an advantage, for example, that a specified maximum amount that cannot be exceeded during a given period of time can be defined for the calling costs.
One system of the above-described kind is disclosed in U.S. Pat. No. 5,408,519. When a subscriber wishes to place a call, he dials the system number, whereafter the system prompts the subscriber to enter an account number and the destination telephone number. The system then calculates the cost per time unit of the call, and the maximum duration for a call to that destination, i.e., how long a call can be placed based on the current account balance. The system also allows the balance of the account to be increased with a credit card in connection with the call, or the system may automatically renew the account balance on a regular basis or each time the balance falls below a given amount. The system charges the increased amount using the credit card data of the subscriber.
If it is desired to maintain a service of the above kind in a telephone network on an exchange-specific basis, tariff data must be maintained at every exchange of the network, and tariff changes must be updated at each exchange.
This drawback can be resolved by keeping subscriber-specific accounts at a centralized point in the network. In that case, however, it will present a problem how tariff changes during the connections are observed in order for it to be possible to decrement the balance of the subscriber at the correct tariff in each case.
This problem could in principle be solved in such a way that when tariff changes are impending, the subscribers will not be granted connection time in accordance with the maximum time available in the account, but call time will only be granted up to the point of the tariff change, and thus the tariff change can be observed immediately when it takes effect. This means, however, that additional time must be given to each ongoing call (in the same tariff class) contemporaneously, which may produce a considerable instantaneous peak in the load on the system.